For the last two years I have just been using a reflective folding cover that I put in the windshield everyday. It works well and I haven't had any problems. The best protection in my opinion is keeping the leather dash conditioned. That will protect the dash from UV, etc. and a reflector will protect the rest of the interior.
You can also buy something like this, but I am not sure how well it would work. With the top down it could flap around and scratch the paint.
http://www.roadstersolutions.com/page19.html